Zoom lenses generally are formed of a plurality of groups (also referred to as units) of one or more lens elements. It has been known to make three-unit zoom lenses with a positive front lens unit, a positive middle lens unit, and a negative rear lens unit. However, because of the desire to reduce the size, weight and costs of cameras with zoom lenses, efforts are being made to create more compact zoom lenses with a small number of lens elements and a minimum number of independent zooming lens units, while still producing high quality images over a large focal length range.
More specifically, a zoom lens must maintain high image quality in each zoom position and retain a flat field over a range of field angles. In addition, it is desirable to have a minimum front vertex to image distance in the telephoto position. This provides a small package size and decreases the overall zoom lens volume. It is also desirable to have a minimum number of lens elements and a minimal number of separate zooming lens units in order to reduce cost and to decrease mechanical complexity. However, the requirement to produce a compact zoom lens with a minimum number of independently movable zooming lens units and a minimum number of lens elements often conflicts with the requirement to produce high quality images over a large focal length range. This conflict occurs because when (i) the air spaces between lens units are reduced and (ii) the front vertex to the image plane distance is also reduced (provided that the optical power of the lens units does not change), the lens units can not move far enough during zooming. The lack of travel distance, in turn, decreases the zoom ratio. The zoom ratio ZR is measured by the ratio of focal lengths, i.e. ZR=f.sub.t /f.sub.w, is the focal length of the zoom lens in the wide-angle mode and f.sub.t is the focal length of the zoom lens in the telephoto position. If the optical power of the lens units increases (without additional lens elements) in order to provide a larger zoom ratio, lens surfaces become stronger, compromising the performance of the zoom lens. In addition, the shorter the focal length of the zoom lens in the wide angle mode, the more difficult it is to control field aberrations such as coma, astigmatism and field curvature. Controlling these aberrations in a zoom lens that has a minimum number of independently movable lens units and provides a zoom ratio of over 2.5.times. often results in excess of axial color aberration in the wide angle mode and in lateral color aberration in both wide angle and the telephoto modes. These color aberrations can be controlled by additional lens elements, but this further increases the complexity and size of a zoom lens.
Thus, as the optical powers of lens units increase in order to increase the zoom ratio to about 3.times., it becomes more difficult to maintain the optical performance of the zoom lens. Because of this, zoom lenses with large zoom ratios require larger numbers of lens elements. This, in turn, increases the cost of the zoom lenses and often makes the zoom lens less compact. As stated above, if the number of lens elements in a zoom lens is reduced, the performance of the zoom lens suffers.
The compactness of the zoom lens can also be measured by the ratio L.sub.v to f.sub.t, where L.sub.v is the distance from the front vertex of the zoom lens to the film plane in the telephoto position. The smaller the ratio, the more compact the zoom lens. However, as stated above, the compactness of the zoom lens should not come as a result of reduced zoom ratio or reduced optical performance.
The following U.S. patents all describe zoom lenses which have three lens units of positive, positive and negative optical power, respectfully.
U.S. Pat. No. 5,825,556 and No. 5,726,810 disclose compact zoom lenses. These zoom lenses provide an excellent performance with only 5 lens elements and a minimum amount of independently moving lens units. More specifically, the front and the rear lens units move together for zooming and the middle lens unit moves independently. The compactness ratio L.sub.v /f.sub.t, of the zoom lenses disclosed in these patents are about 0.9. The disclosed zoom lenses provide a zoom ratio of 2.times..
U.S. Pat. No. 5,715,096 also discloses a compact three-unit zoom lenses. However, during zooming, all lens units move independently of one another-i.e., they move at different rates. These zoom lenses provide a zoom ratio of over 3.times. (the focal length f.sub.w in the wide angle mode is 29 mm and its focal length f.sub.t in the telephoto mode is 96.5 mm). The compactness ratios L.sub.v /f.sub.t, of the zoom lenses disclosed in this patent are about 0.9. However these zoom lenses require more complex driving means than the zoom lenses disclosed in the U.S. Pat. Nos. 5,825,556 and 5,726,810. The disclosed zoom lenses have lateral color aberration. The maximum amount of lateral color is about 0.02, 0.03, 0.02 in a first, second and third embodiment, respectively.
U.S. Pat. No. 4,726,668, issued Feb. 23, 1988, discloses a zoom lens with three lens units of positive, positive and negative optical powers, respectfully. An aperture stop is located within the middle lens unit. The front and the rear lens units move independently of each other while the second unit, which contains an aperture stop, is held stationary during zooming. This zoom lens has six lens elements and the total length from the front vertex of the zoom lens to its rear vertex is 46.95 mm at the telephoto position. At the wide-angle position, the maximum amount of distortion is 5% and the amount of astigmatism is 0.8 millimeters. In the telephoto position, the distortion amount is again about 5% and the maximum astigmatism is greater than 0.8 millimeters. It is desirable to have a zoom lens that has better performance with respect to astigmatism and distortion, fewer lens elements and that is more compact.
U.S. Pat. No. 5,353,160, issued Oct. 4, 1994, discloses a zoom lens which has either four or five lens elements arranged into three movable lens units. The zoom lens has a zoom ratio ZR=f.sub.t/f.sub.w of only 2.0, and its compactness ratio L.sub.v /f.sub.t is 0.93.
U.S. Pat. No. 5,325,235, issued Jun. 28, 1994, has relatively few lens elements (four). Its zoom ratio ZR is only 1.6 and its compactness ratio is about 1.0 (for example, it is 0.96 and 1.13 in the embodiments 3 and 5, respectfully). Thus, this zoom lens is even less compact and has an even smaller zoom ratio than the zoom lens described in U.S. Pat. No. 5,353,160.
U.S. Pat. No. 5,353,159, issued Oct. 4, 1994, discloses a zoom lens with three lens units that move during zooming. The zoom lens has a zoom ratio ZR of only 1.87, and a compactness ratio of about 1.01. This zoom lens has 8 or 9 lens elements.
U.S. Pat. No. 5,216,547, issued Jun. 1, 1993, discloses a zoom lens with three lens units that move during zooming. The zoom lens has a zoom ratio ZR of only 1.87, and a compactness ratio of about 0.94. This zoom lens has 7 lens elements.
Other patents also provide a three unit zoom lenses, but they are either substantially less compact than the lens systems of the above described U.S. patents, or have a substantially greater number of lens elements or have a substantially smaller zoom ratio.